package bouncer

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"strconv"
	"strings"

	"github.com/golang-jwt/jwt/v5"
)

const (
	msgTokenNotProvided = "Token not provided"
	msgTokenExpired     = "Token has expired"
	msgUserNotFound     = "User not found"
	msgBadSignature     = "Token Signature could not be verified."
	msgMalformed        = "Wrong number of segments"
	msgRequiredClaims   = "JWT payload does not contain the required claims"
)

// UserProvider loads a persisted user by JWT subject.
type UserProvider interface {
	FindByID(ctx context.Context, id uint) (*Principal, error)
}

// Middleware validates a pinned HS256 bearer token and loads the user.
func Middleware(secret string, users UserProvider) func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			raw, err := bearerToken(r)
			if err != nil {
				write401(w, err.Error())
				return
			}
			sub, err := Verify(raw, secret)
			if err != nil {
				write401(w, err.Error())
				return
			}
			id, err := strconv.ParseUint(sub, 10, 64)
			if err != nil || id == 0 {
				write401(w, msgUserNotFound)
				return
			}
			if users == nil {
				write401(w, msgUserNotFound)
				return
			}
			user, err := users.FindByID(r.Context(), uint(id))
			if err != nil {
				write401(w, "Authentication error")
				return
			}
			if user == nil {
				write401(w, msgUserNotFound)
				return
			}
			next.ServeHTTP(w, r.WithContext(WithUser(r.Context(), user)))
		})
	}
}

type jwtGuard struct {
	secret string
	users  UserProvider
}

var (
	_ Guard              = (*jwtGuard)(nil)
	_ UnauthorizedWriter = (*jwtGuard)(nil)
)

// NewJWTGuard adapts the existing bearerToken -> Verify -> users.FindByID
// chain (identical to Middleware's body) into a Guard + UnauthorizedWriter,
// so Registry.Middleware("jwt") is byte-identical to bouncer.Middleware.
func NewJWTGuard(secret string, users UserProvider) Guard {
	return &jwtGuard{secret: secret, users: users}
}

func (g *jwtGuard) Authenticate(r *http.Request) (*Principal, error) {
	raw, err := bearerToken(r)
	if err != nil {
		return nil, err
	}
	sub, err := Verify(raw, g.secret)
	if err != nil {
		return nil, err
	}
	id, err := strconv.ParseUint(sub, 10, 64)
	if err != nil || id == 0 {
		return nil, errors.New(msgUserNotFound)
	}
	if g.users == nil {
		return nil, errors.New(msgUserNotFound)
	}
	user, err := g.users.FindByID(r.Context(), uint(id))
	if err != nil {
		return nil, errors.New("Authentication error")
	}
	if user == nil {
		return nil, errors.New(msgUserNotFound)
	}
	return user, nil
}

func (g *jwtGuard) WriteUnauthorized(w http.ResponseWriter, err error) {
	write401(w, err.Error())
}

// Verify parses a token with HS256 pinned and a required exp and sub.
func Verify(tokenString, secret string) (string, error) {
	if strings.TrimSpace(secret) == "" {
		return "", fmt.Errorf("bouncer: jwt secret is empty")
	}
	parser := jwt.NewParser(jwt.WithValidMethods([]string{"HS256"}), jwt.WithExpirationRequired())
	claims := jwt.MapClaims{}
	_, err := parser.ParseWithClaims(tokenString, claims, func(t *jwt.Token) (any, error) {
		return []byte(secret), nil
	})
	if err != nil {
		return "", mapJWTError(err)
	}
	sub := subject(claims)
	if sub == "" {
		return "", errors.New(msgRequiredClaims)
	}
	return sub, nil
}

func bearerToken(r *http.Request) (string, error) {
	h := strings.TrimSpace(r.Header.Get("Authorization"))
	if h == "" {
		return "", errors.New(msgTokenNotProvided)
	}
	token, ok := strings.CutPrefix(h, "Bearer ")
	token = strings.TrimSpace(token)
	if !ok || token == "" {
		return "", errors.New(msgTokenNotProvided)
	}
	return token, nil
}

func subject(claims jwt.MapClaims) string {
	switch v := claims["sub"].(type) {
	case string:
		return strings.TrimSpace(v)
	case float64:
		if v <= 0 {
			return ""
		}
		return strconv.FormatInt(int64(v), 10)
	case json.Number:
		return strings.TrimSpace(v.String())
	default:
		return ""
	}
}

func mapJWTError(err error) error {
	switch {
	case errors.Is(err, jwt.ErrTokenExpired):
		return errors.New(msgTokenExpired)
	case errors.Is(err, jwt.ErrTokenSignatureInvalid), errors.Is(err, jwt.ErrTokenUnverifiable):
		return errors.New(msgBadSignature)
	case errors.Is(err, jwt.ErrTokenMalformed):
		return errors.New(msgMalformed)
	default:
		msg := err.Error()
		if strings.Contains(strings.ToLower(msg), "expired") {
			return errors.New(msgTokenExpired)
		}
		if strings.Contains(strings.ToLower(msg), "malformed") || strings.Contains(msg, "segment") {
			return errors.New(msgMalformed)
		}
		return errors.New(msgBadSignature)
	}
}

func write401(w http.ResponseWriter, message string) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(http.StatusUnauthorized)
	_ = json.NewEncoder(w).Encode(map[string]any{"error": true, "message": message})
}